1) Field of the Invention
This invention relates to a drive power distribution control system for a vehicle, which is suitable for use in distributing drive power in an automotive vehicle, for example, in distributing drive power to a left and right drive wheels in a 4WD or 2WD automotive vehicle, in distributing drive power to front and rear drive wheels in a 4WD automotive vehicle, or in distributing drive power by transferring drive power between a left and right non-drive wheels (as opposed to drive wheels) in a 2WD automotive vehicle.
2) Description of the Related Art
In recent years, active developments are underway on 4WD automotive vehicles (hereinafter called "4WD vehicles"). Developments of automotive vehicles of the full-time four wheel drive system, which permit positive control of distribution of torque (drive power) between front and rear wheels, are carried out in various ways.
For such full-time 4WD vehicles, front/rear drive power distribution systems have been developed, which make it possible to control the distribution of drive torque (hereinafter called merely "distribution of torque" or "distribution of drive power") between front and rear drive wheels. The front/rear drive power distribution systems include, for example, a viscous coupling unit (VCU) and a hydraulic coupling unit (HCU).
In each of the aforementioned conventional front/rear drive power distribution systems, however, the distribution of torque between the front and rear wheels is generally controlled by transmitting drive force from the side of the wheels rotating at a higher speed to the side of the wheels rotating at a lower speed to reduce a differential motion between the front and rear wheels when the differential motion occurs. This torque distribution control is however conducted corresponding to each differential motion in accordance with preset characteristics of the system, for example, corresponding to other control elements, so that it is not designed to positively control the distribution of torque.
For entire automotive vehicles not limited to 4WD vehicles but including 2WD vehicles, developments have been made on left/right drive power distribution control systems which control the distribution of torque to be transmitted to a left and right wheels, respectively.
If such left/right drive power distribution control systems are taken in broad sense, conventional normal differentials and LSDs (limited slip differentials) including electronically-controlled LSDs may be contemplated. It is however to be noted that such conventional differentials do not positively distribute torque and cannot freely distribute torque to a left and right wheels at a desired ratio.
In parallel with torque distribution control systems between front and rear wheels, development of a system capable of controlling the distribution of torque between a left and right wheels is also desired. In this case, the control is directed not only to the distribution of torque between left and right drive wheels in a 4WD vehicle but also to the distribution of torque between a left and right drive wheels in a 2WD vehicle.
If the distribution of torque is taken in such broad sense that it covers not only the distribution of torque outputted from an engine but also the transmission of torque which takes place through transfer of drive power between a left and right axle portions, it may be contemplated to control the distribution of torque between a left and right non-drive wheels (as opposed to drive wheels) in a 2WD vehicle.
Neither the left non-drive wheel nor the right non-drive wheel receives drive power from the engine. If it is however possible to realize transfer of drive power from one of these non-drive wheels to the other, brake force can be developed on the side of the former non-drive wheel and drive power can be produced on the side of the latter non-drive wheel. Accordingly, it becomes possible to control the distribution of torque (including negative drive power, namely, brake force) between the left and right non-drive wheels.
Further, desired as such a drive power distribution control system for a vehicle is one that can perform the distribution of torque without inducing any large torque loss or energy loss.